There is an increasing concern to reduce the consumption of resources, both at a domestic level in residential buildings, and at a commercial level in offices, shops, factories and so forth. The reasons for this are both to save costs and also because of concerns for the environment, such as to reduce CO2 emissions, and to conserve finite resources such as coal, gas and oil.
Conventionally, consumers receive bills from utility companies which may indicate the quantity of the utility used since the last bill, for example monthly or quarterly, based on periodic meter readings or even based on estimates of consumption since the last meter reading. For example, in the case of electricity supply, the information is presented to the consumer in terms of the number of kilowatt hours of electrical energy that has been used, which is meaningless to many people, and gives very little idea about how they are actually using the energy and where they can cut back. Studies have shown that the effect of providing consumers with real-time detailed information about the energy they are using is that their consumption reduces by up to 20%. In order to provide this information, it is necessary to identify where the energy drawn from this supply is ending up, i.e. which appliances are being used, how much and when. It is a problem to provide this information.
Devices are known which can be plugged into a conventional electricity outlet socket which can monitor the energy consumption by a particular appliance plugged into that socket. However, this information is inconvenient to obtain, and for fully monitoring the consumption at a particular site, such as a house, a separate metering device would have to be plugged into every socket to monitor every appliance, and it is generally not possible to connect such metering devices to permanently-wired appliances, such as cookers, which are typically some of the largest consumers of energy.
As an alternative to attaching a metering device to each appliance, Non-Intrusive Load Monitors (NILMs) have been developed. Many NILM implementations work by tracking the steady state power drawn by the load and measuring significant changes which correspond to appliances turning on and off. For example, following the turn on of a 100 W incandescent light, one would observe a change in power of around 100 W. A NILM would register this change and search through its database to see whether this appliance ‘signature’ matched those of a previously observed appliance. If so, then it would infer that the appliance had turned on. If not, then it would infer that a previously unknown appliance had turned on for the first time. An overview of NILM technology is found in U.S. Pat. No. 4,858,141 (Hart et al.).
The approach of measuring steady state powers generally works well, but it does have problems associated with it. Such problems include (1) certain appliances do not have a repeatable steady state power (e.g. the power consumed by a motor varies with the loading on that motor); (2) some appliances do not settle to a steady state power (e.g. the constant movement of the clothes inside a tumble dryer cause a time varying load on the motor and thus an oscillating power consumption); and (3) it is hard to differentiate between different load types (e.g. a motor and a fluorescent light) using only the steady state information.
In an attempt to overcome this third problem, U.S. Pat. No. 5,483,153 (Leeb and Kirtley) make use of additional information contained in the transient signature of the load. Thus, U.S. Pat. No. 5,483,153 discloses a ‘transient event detector’ which detects significant variations (so-called ‘v-sections’) in the power supply and attempts to match the shape of the observed v-section with previously observed v-sections by using transversal filters. Induction motors may be distinguished from fluorescent lamps in this way. Additionally, U.S. Pat. No. 5,483,153 identifies that certain load types (e.g. induction motors) have similarly shaped v-sections and thus builds into the system an ability to scale the reference v-section in both time and amplitude to attempt to match a load to a previously unidentified load. However, the method of U.S. Pat. No. 5,483,153 requires that the transient turn-on profiles are repeatable, whereas the torque of an induction motor is not constant with speed and so a non linear stretching in time of the transient can occur for the same motor with different loads. Furthermore, the v-section solution of U.S. Pat. No. 5,483,153 does not deal with problems (1) and (2) identified above.
The present invention aims to alleviate, at least partially, one or more of the above problems.